Extruders for processing plastics require a relative large amount of energy in operation, and the required precise temperature control is frequently not possible, even if the extruders have an electric heating system and cooling coils for a cooling liquid. It has also been suggested to produce thermoplastics on extruders, but hitherto that type of production could not become successfully established, due to the difficulties involved. However, there is a need for versatile reactors which can be used for various chemical reactions, are designed for low to medium production runs and can be re-equipped at low cost.